Compact source lamp



nited tate fO COMPACT SOURCE LAMP Walter Adolf Haidinger, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., Irvington on Hudson, N.Y.

Application February 18, 1957, Serial No. 640340 Claims priority, application Netherlands February 24, 1956 6 Claims. (CI. 313-184) This invention relates to high pressure electric discharge lamps for producing very bright illummaton emanating from a very small Volume Wlthm the tube envelope. Such lamps are known as compact source lamps. D

Compact source lamps are frequently used n optcal apparatus because of their ability to cast very sharp Shadows due to the fact that they have a high brghtness. For example, such lamps are used in the television cathode-ray tube industry -to produce color television tubes of the shadowmask type. An essential step n making shadow-mask tubes consists in casting on a photosensitized plate the Shadows of about 200,000 closely-spaced holes in a thin mask. The Shadows must be sharp and absolutely must not move about during the exposure of the photosensitive material. Anything which causes the minute luminescent point to move around wthn the lamp also causes the Shadows to move around and to appear to be less sharp. D

The primary object of the present mventon s to mprove upon the stability of location of the lumnescent source while preserving a fairly long life for the lamp.

The invention consists of a transparent envelope filled with compressed gas and having two electrodes extending into it from opposite sides and aligned Wlth each other on a common axis, the spacing of said electrodes being small as compared with the diameter of the lamp. At least one of the electrodes has a conical depresson on it. At the apex, or bottom, of the conical depression is a cylindrical hole which is bored still deeper into sad electrode and which may contain electron emissve material. The dscharge current of the lamp is set to a value exceeding that at which the dscharge involving a focal spot passes over into a dscharge without focal spot.

The invention and a preferred mode of operation of it will be further described in connection with the drawing, which shows a cross sectional view of a compact source lamp that incorporates the features of the invention.

The lamp includes a transparent envelope 1, preferably of quartz, into which a pair of cylindrical electrodes 2 and 3 are sealed. The end of electrode 2 tapers oif into a conical point, while the facing end of the other cylindrical electrode 3 is provided with a conical depression 4. The included angle of the conical depression is preferably about 120, although the lamp will work Satisfactorily with angles of about 30" to l50. At the apex of this depression there is a cylindrical hole 5, the diameter of which is about A to /2 the diameter of electrode 3 and which extends axially into the cathode cylinder for a length that preferably exceeds its diameter. This hole may remain empty, or, if desired, it may be filled with electron emissive material such as thorium oxide. The envelope 1 is filled with a rare gas at a pressure several times as great as atmospheric pressure.

In Operating the lamp it has been found to be desirable 2,887,6o3 Patented May p 19, 1959 tooperate it with the electrodes 2 and 3 in a vertical i position with electrode 2 on top as shown in the drawing and also to supply sufl'icient dscharge current to the electrodes to cause the dischargeto operate without a focal spot. In this way it has been found possible to suppress a gas jet which otherwise would be formed at right angles to the cathode within the envelope 1 and which ordinarily would cause the luminescent Volume to bob around instead of staying in a fixed location. By virtue of the conical depression 4 in the end of electrode 3, it has been found that the temperature of the conical surface is exceedingly uniform and is lower than has been obtained with electrodes used in prior compact source lamps. A favorable result of this lower operating temperature is that the form of the conical cavity is preserved for a considerable time, thereby increasing the useful life of the lamp.

The cylindrical hole 5 has an unexpectcd efiect on the dscharge; it decreases the current strength required to produce a dscharge without a focal spot.

The lamp may be operated in series with a ballastimpedance either on direct current or alternating current, and in the case of alternating current operation and a dscharge without a focal spot, substantially no reignition peak of the are voltages occurs.

Dimensions for one embodment of the lamp which has been found to work quite well are given herewith in order that the invention may be more readily put into use, :although it will be understood that these dimensions are not to be considered as limiting the invention. A spherical quartz envelope 1 having an inside diameter of approximately 20 mm. and filled with Xenon at a pressure of approximately 10 atmospheres at room temperature was provided with tungsten electrodes 2 and 3, the ends of which were spaced apart by approximately 2.5 mm. The diameter of electrode 2 Was 1.6 mm. and the included angle of its conical point was about Electrode 3 had a diameter of about 1.4 mm. and the included `angle of the conical depression 4 was approximately The diameter of hole 5 was 5 mm. and its length was about 2 mm.

The lamp operated at a voltage of about 18 volts and at an alternating current of about 9 amperes. The discharge was quiet and the focal spot disappeared at a current above about 7 amperes. The life, measured to the time when the luminescence had reduced to about 75% of its inital value, was approximately 500 operating hours.

What is claimed is:

l. A high-pressure electric dscharge lamp adapted to operate without a focal spot comprising a light transmissible envelope, an inert gas filling said envelope at a pressure of several atmospheres, a pair of axially aligned spaced conductve electrodes within said envelope, the spacing of said electrodes being small compared with the diameter of the envelope, one of said electrodes having a conical depression the base of which faces the other electrode and the apeX of which lies within and on the axis of said electrode, said electrode also having an axial cylindrical recess extending from the apex of said conical depression into said electrode for reducing the current strength at which a dscharge without a focal spot is produced, and the other of said electrodes having a conical tip whereby an are-dscharge without a focal spot is produced between said electrodes which is viewed at right angles to the axis of said electrodes.

2. A lamp as claimed in claim 1 wherein the included angle of said conical depression is between about 30 and about 3. 3. A lamp as claimed in claim 1 in which the included angle of said conical depression is approximately 120.

4. A lamp -as claimed in claim 1 in which said cylindrical hole is filled with electron emissive material.

5. A lamp as claimed in claim 1 in which the axial length of said cylnd'ical hQle is greate' than its diameter. 6. A lamp as claimed in claim 5 in which the inside diameter of'said cylindrcal hole is between /a and /2 the diameter of said other electrode.

References Cited in the file of this patent UNITED STATES PATENTS Bassett Sept. 5, Bassett June 14, Buckingham et al. Nov. 9, Buckingham Dec. 8, Noel Aug. 30,

Morin Dec. 27, 

